The present invention relates to a magneto-optical recording medium for recording drives. More particularly, it is concerned with a magneto-optical recording medium having double magnetic layers and being capable of over-writing with a single laser beam.
A conventional magneto-optical recording medium having double magnetic layers is disclosed in Japanese Patent Laid-open No. 175948/1987. It has a structure as shown in FIG. 2. It is composed of a transparent substrate 1 (glass or the like) having tracking grooves and four thin layers consecutively formed thereon, each designated as a first dielectric layer 2 of silicon nitride (ca. 90 nm thick), a recording layer 3 of TbFeCo (ca. 100 nm thick), a supporting layer 4 of TbDyFeCo (ca. 150 nm thick), and a second dielectric layer 5 of silicon nitride (ca. 200 nm thick).
The four thin layers have the following functions. The first dielectric layer 2 causes the laser light incident on the transparent substrate 1 to undergo multiple reflection in the layer, so as to increase the angle of rotation (Kerr rotation) of the plane of polarized light by the recording layer 3. The recording layer 3 is thick enough (ca. 100 nm) to prevent the transmission of light, so that the light does not reach the supporting layer 4 and the plane of polarized light rotates in response to the direction of magnetization in the recording layer 3. The second dielectric layer 5 protects the recording layer 3 and the supporting layer 4 from corrosion (e.g., oxidation). The supporting layer 4 is in exchange coupling with the recording layer 3 through exchange interaction.
The recording layer 3 and supporting layer 4 are formed such that the former has a lower Curie temperature than has the latter and the former has a greater coercive force than has the latter at room temperature. For this reason, the direction 14a of magnetization in the supporting layer 4 align irrespective of the direction 14b of magnetization in the recording layer 3, upon mere application of the initializing field by a permanent magnet, as shown in FIGS. 8(a) and (b).
When the recording medium of this type is irradiated with a laser beam of comparatively low intensity, the recording layer 3 heats up to a temperature (T) which is higher than the Curie temperature. (FIG. 8(c)) Therefore, in the subsequent cooling process, the direction 14a of magnetization in the recording layer 3 aligns with the direction 14b of magnetization in the supporting layer 4. (FIG. 8(e)) On the other hand, when the recording medium is irradiated with a laser beam of comparatively high intensity, the supporting layer 4 heats up to a temperature (T) which is higher than the Curie temperature. (FIG. 8(d)) Therefore, in the subsequent cooling process, the direction 14a of magnetization in the supporting layer 4 aligns with the direction 13 of the recording field externally applied by means of a permanent magnet. (FIG. 8(f)) Upon further cooling, the direction 14b of magnetization in the recording layer 3 aligns with the direction 14a of magnetization in the supporting layer 4. (FIG. 8(g)) Thus the direction of magnetization in the recording layer 3 can be reversed as desired according as the intensity of laser beam is modulated. This is the fundamental mechanism which permits overwriting with a single laser beam. A detailed description of this method will be found in Japanese Patent Laid-open No. 175948/1987.
The conventional recording medium mentioned above has several disadvantages arising from its thick recording layer 3. The thick recording layer 3 needs a laser beam of high intensity for recording on account of its high heat capacity. Also, the thick recording layer 3 gets hot sharply at its center when irradiated with a laser beam, as shown in FIG. 9. This local heating degrades the magnetic properties and read-out characteristics of the recording layer 3 and supporting layer 4 after repeated overwriting. Moreover, the thick recording layer 3 only permits the use of Kerr rotation due to surface reflection as the magneto-optical effect. (Kerr rotation is not large enough to secure a sufficient carrier-to-noise ratio.)
On the other hand, reducing the thickness of the recording layer 3 causes the supporting layer 4 to produce reflected rays which adversely affect read-out. In other words, the supporting layer 4 changes the angle of rotation of the plane of polarization of laser light depending on the direction of its magnetization. Therefore, the use of double magnetic layers for overwriting might bring about a situation in which the direction of magnetization in the supporting layer 4 is opposite at the time of reproduction from that immediately after overwriting. This situation makes it difficult to perform the read-out for verification immediately after overwriting using double beams. The read-out for verification is possible only after the disk has made a turn during which the direction of magnetization in the supporting layer 4 aligns. In other words, the read-out for verification needs an additional turn of the disk (and hence the recording needs two turns of the disk in total). This leads to a slow data processing speed.
Another disadvantage of the conventional recording medium is that the supporting layer should be thicker than the recording layer, with the total thickness of the magnetic layers being 150-300 nm. This leads to a low recording sensitivity.
Further another disadvantage of the conventional recording medium is that the Curie temperature of the recording layer 3 should be lower than that of the supporting layer 4. This makes it impossible to increase the Kerr rotation that occurs when the recording layer 3 is irradiated with a laser beam. Therefore, the conventional recording medium has a low C/N ratio. (Note that there is a relationship between the C/N ratio and the Curie temperature as shown in FIG. 22.) If the Curie temperature of the recording layer 3 is to be increased, it is necessary to increase the Curie temperature of the supporting layer 4, too. However, the supporting layer 4 with an excessively high Curie temperature reduces the recording sensitivity.